Revise the condition to add specs

[saschanaz]

https://github.com/Microsoft/TSJS-lib-generator/blob/8754e3d9eef27794b2475d21e9a80221c74010e5/README.md#L56-L59

Currently the readme says working drafts can't be in lib.d.ts, but we recently merged several draft-level types into lib.d.ts because migration from DT to lib.d.ts often creates unexpected type errors. (microsoft/TypeScript#18480 (comment))

Thus, the readme should reflect the recent situation to prevent any confusion.